Many telephone service providers (TSP's) offer a network-based voicemail service for their customers, so that a caller can leave a voicemail message for a customer of the TSP. When provided by a Local Exchange Carrier (LEC), these voicemail services typically utilize Simplified Message Desk Interface (SMDI) links to transmit a message-waiting notification between the LEC's switch and the called subscriber's line. In one implementation, a message-waiting notification creates a stutter dial tone heard by the customer when they attempt to make a call. In another implementation, a message-waiting notification actuates a Message Waiting Indicator (MWI), which is a small LED usually provided on either the customer's telephone or on a small box connected to the telephone via an RJ-11 connector.
TSP's other than the customer's LEC are usually at a disadvantage in providing message waiting notification to the customer since SMDI links can be very costly to install. Such links typically require a dedicated connection from the non-LEC TSP to the local switch serving the customer of the non-LEC TSP, that switch operated by the customer's LEC. Moreover, SMDI links can be very difficult to maintain, because, among other reasons, the non-LEC TSP can need the cooperation of the LEC, which might be a business competitor of the non-LEC TSP.